How to force a node group input field to display a slider?
You cannot change a field to display a slider once it has been initialized as a non slider. But it works both ways, if you initialize a field with a slider (Such as a factor input in a mix RGB), and then change the input to something else, the input will keep its slider.
Explanation :

CTRL + G to group several nodes.
You can get rid of the Mix RGB afterwards.
To remap the min and max values of the slider, when inside the node group, open the &quot;N&quot; panel with N or click on the left facing arrow in the top right of the screen and select your slider input. You can then change the min and max values.

Note : It also works for colors for instance. In this case the RGB channels will get replaced by the factor value. Although it is considered bad practice to link different colors inputs/outputs.